Optical transmission systems for optical communications typically include a pair of network nodes connected by an optical waveguide (i.e. fiber) link. Within each network node, optical signals are converted into electrical signals for signal regeneration and/or routing. Exemplary network nodes of this type include add-drop-multiplexers (ADMs), routers, and cross-connects. The optical link between the network nodes typically comprises multiple spans (e.g. of 40-60 km in length) interconnected by optical devices such as Erbium amplifiers which operate to extend the signal reach within the link.
Traditionally, optical devices have been implemented as discrete components coupled between adjoining spans. Thus, for example, optical signals are progressively attenuated as they propagate through a span, amplified by an optical amplifier prior to being launched into the next adjoining span. A disadvantage of discrete optical devices is that the link is discontinuous at the device, in that the waveguide must be severed and coupled to the optical device. This discontinuity leads to problems such as signal reflection and scattering, both of which tend to degrade the performance of the link. An additional disadvantage of the use of discrete optical devices is that power (typically electrical) must be supplied to the optical device, thereby increasing equipment costs. It is therefore desirable to minimize the number of discrete optical devices within a link.
One method of reducing the number discrete optical devices within a link is to pre-amplify data signals within the waveguide upstream of each device, and thereby increase the length of each span. This can be accomplished by exploiting Raman scattering phenomena to couple power from a pump laser to data signals within the waveguide. Typically, the pump laser is injected into the waveguide near the output end of the span (that is, near the discrete optical device which is receiving data signals) and propagates in a reverse direction, opposite to the propagation direction of the data signals. Optical pre-amplification in this manner is typically referred to as “Raman amplification” or “Raman pumping”, and has the effect of increasing the signal to noise (S/N) ratio at the input of the receiving optical device, thereby enabling an increased span length between adjacent optical devices within a link.
A characteristic feature of Raman pumping is that the signal gain is distributed within the span, typically within a gain region extending up to 10 km or more (depending principally on the waveguide properties and the pump power) from the injection point of the pump laser. In general, gain due to Raman pumping is a maximum at the injection point of the pump laser, and decays exponentially with distance away from the injection point. Thus the length of the gain region is normally based on an arbitrary gain threshold, rather than any distinct transition of signal gain within the waveguide. Furthermore, signal gain is a function of both pump injection power and position within the gain region, and it is difficult to determine the distribution of gain as a function of length.
In particular, it is possible to measure data signal power at the extreme ends of the waveguide, and therefore obtain net attenuation or gain through the waveguide. However, without a measure of the signal power prior to entering the gain region, the gain produced by Raman pumping cannot be determined.
U.S. Pat. No. 6,072,614 (Roberts) teaches a method of determining signal attenuation as a function of length along a waveguide. However, this technique relies on detection of counter-propagating signals due to various scattering phenomena (e.g. stimulated Brillouin scattering and Rayleigh scattering) and cannot provide any information concerning gain due to Raman pumping. In principle, the method of U.S. Pat. No. 6,072,614 can be used to estimate signal power prior to entering the gain region. However, the boundary of the gain region is generally indistinct, and its location may not be known in advance.
One technique for determining gain distribution within a waveguide is known as the “cut-back” method. In this technique, the waveguide is progressively shortened by cutting pieces from the input end of the waveguide, and the resulting changes in measured signal power at the receiving end of the waveguide used to determine gain as a function of waveguide length. This technique is suitable for laboratory testing. However, it is clearly not suitable for installed links, which must necessarily be provisioned with a fixed geographical length. Furthermore, since this technique necessarily results in the destruction of the waveguide, its results can only be used as an estimate of the performance of installed fibers, as manufacturing variations may produce differing optical performance in different fibers.
Accordingly, a method and system for non-destructively monitoring distributed gain as a function of length along a waveguide remains highly desirable.